There has been conventionally known a technique for simulating acoustic effects of a stereophonic signal (e.g., a 5.1 channel) using a front loudspeaker. According to this technique, a listener is enabled to perceive a stereophonic effect without requiring a surround speaker, an earphone, a headphone, and others. For examples, a listener can feel auditory lateralization behind himself/herself by using two front loudspeakers. Such a technique is based on a control policy for faithfully reproducing a binaural acoustic signal (or an acoustic signal coming from a virtual acoustic source) in both ears of a listener using a head-related transfer function.
As problems of such a technique, there are known a deterioration in acoustic quality due to deficiency in dynamic range, an increase in a hardware scale or a reduction in processing speed due to a signal processing load using a head-related transfer function, a localization of a binaural position at which auditory lateralization can be obtained, and others. For example, according to many conventional techniques, desired stereophonic effects can be achieved only when one listener is located at a vertex (a sweet spot) of a regular triangle having a line connecting two front loudspeakers as a bottom side. If a binaural position of the listener deviates from this sweet spot (e.g., approximately several tens of cm), the head-related transfer function fluctuates, and hence a binaural acoustic signal (or an acoustic signal coming from a virtual acoustic source) is not faithfully reproduced. That is, desired acoustic effects cannot be achieved. Therefore, the above-described control policy has a problem that it lacks robustness with respect to a fluctuation in binaural position of a listener.